I'm now getting:

ELF: call to elf_begin() failed
makeself.exe: Unable to parse ELF file "testlibm.elf"

Tell me if you want more tests, or maybe you now prefer to invest efforts on the bflt loader.

I also have this error.
The error is in xread(), which is in <libelf/input.c>.

static int
xread(int fd, char *buffer, size_t len) {
    //...
    while (done < len) {
n = read(fd, buffer + done, len - done);
if (n == 0) {
    /* premature end of file */
            // HERE
    return -1;
}
        //...
}
As ExtendeD said, tell me if your want more tests, or if you wish me to use the bflt loader.